Traditionally, a discharge lamp lighting device for lighting high pressure discharge lamps such as metal halide lamps or the like. Such a discharge lamp lighting device employs a square wave lighting technique to avoid an acoustic resonance phenomenon and has been used for lighting the light sources of, e.g., a spotlight, a projector and the headlight device of a vehicle.
This kind of discharge lamp lighting device has a DC power source which outputs a DC power, and an inverter which inverts the polarity of the DC power outputted from the DC power source at a predetermined inversion time interval to thereby obtain a square wave AC power and then supplies the square wave AC power to a discharge lamp.
In the above discharge lamp lighting device, when the polarity of the output from the inverter is inverted (hereinafter, referred to simply as “inversion”), the temperature of an electrode of the discharge lamp drops as the output current from the inverter to the discharge lamp is temporarily decreased to thereby make the discharge of the discharge lamp after inversion instable, thus causing flickering or extinction of the discharge lamp or generating electronic noises.
To prevent such problems, e.g., Japanese Patent Laid-open Application Nos. H10-501919 and 2002-110392 discloses a technique of temporarily increasing the output power from the inverter (hereinafter, referred to simply as “output power”) right before or after the inversion. If the output power is increased right before inversion as described in H10-501919, the temperature drop in a discharge lamp is constrained. Also, as described in 2002-110392, an increase in the output power right after inversion contributes to a quick temperature recovery after the temperature drop in an electrode of the discharge lamp. In this way, discharge in the discharge lamp becomes stable, and thus the flickering or the extinction of the discharge lamp, or the electronic noises can be constrained.
However, if the average value in one period of the output power is set too high, electrical stress upon the discharge lamp increases to thereby shorten the life span of the discharge lamp.